Ground effect vehicles have been developed in both fields of aeronautics and marine craft. Ground effect vehicles are those vehicles which receive reduced drag due to the reduction of wing-tip vortices while traveling at low altitudes near ground, and more typically, near water. The closer the wing tip is to the ground or water, the lower the drag.
Ground effect vehicles generally include marine craft and aircraft. The two are typically distinguished by those that can sustain extended flight without the aid of ground effect (aircraft) and those that cannot (marine craft). The International Civil Aviation Organization (ICAO) and International Maritime Organization (IMO), both organizations of the United Nations, jointly exercise jurisdiction over these vehicles. The ICAO and IMO have also united to develop uniform navigation and safety rules for these types of vehicles.
Aircraft typically employ altimeter systems for determining their altitude with respect to sea level and ground level. Altimeters known to those of ordinary skill in the art include barometric altimeters, radar altimeters, forward-looking infrared (FLIR) and other types of on-board sensors. Of these, barometric altimeters are a standard or common technique for measuring the altitude of aircraft. Barometric altimeters operate on the principle that air pressure varies with altitude, so a measurement of absolute atmospheric pressure can be correlated with altitude.
Barometric altimeters are quite accurate with respect to the measurement of atmospheric pressure. However, there can be variations in atmospheric pressure over time at any given location and altitude. Variations result from weather systems and temperature variations. In practice, such variations are accommodated by periodically adjusting the altimeter to account for the local barometric pressure as relayed to the airplane from a fixed ground station of known altitude. This technique is not possible, however, over large expanses of ocean where no barometric stations exist. Furthermore, even if barometric pressure were known exactly, other uncertainties in the barometric system degrade accuracy. Uncertainties include variations in the aircraft's measurement of barometric pressure due to the influence and fluctuation of the ocean surface; variations in the aircraft's configuration (thrust level, flap setting, airspeed and the like); and variations in the height of the ocean surface due to waves, tides, wind and variations in barometric pressure.
Regardless of the type of altimeter, typically, one altimeter is employed and all portions of the aircraft are presumed to be at a single altitude. In contrast, ground effect vehicles flying much closer to the surface of the ocean are much more greatly affected by differences in wave height and terrain height with respect to portions of the aircraft. In prior embodiments of ground effect aircraft, the aircraft were designed to permit structural portions to touch water during routine flight or to even land on water. For example, the Russian Caspian Sea Monster employed pontoons in order to permit the wing tips to skip off the top of the water in routine flight. Accordingly, differences in wave height and terrain height were not a concern for these prior art ground effect vehicles.
Other aircraft employ ground proximity warning systems that generate warning envelopes to alert pilots to minimum flight altitude requirements based on expected ground terrain. Ground proximity warning systems analyze the flight parameters of the aircraft and the terrain based on sensors and databases in order to establish minimal altitudes and alerts. These are most often used in conjunction with runways and landing systems. These systems also employ other sensors including global positioning system, instrument landing systems with algorithms in combination with the altimeter system to determine appropriate minimum altitudes based on flight conditions such as described in U.S. Pat. No. 6,507,289 to Johnson. Some systems also employ feedback from sensors and transmitters employed in and about runway areas to provide feedback to the ground proximity warning systems such as described in U.S. Pat. No. 6,185,486 to Labounsky. Such systems are not easily adapted to altimeter systems of ground effect vehicles to determine minimum altitudes when flying very close to the ocean surface.
For example, as described in U.S. Pat. No. 6,848,650 to Hoisington, et al. future ground effect aircraft are expected to fly within 20 to 50 feet, and minimum altitude requirements will therefore be required to be determined relative to individual portions of the aircraft rather than using single radio altimeters to determine height from one particular point on the aircraft. In particular, the aerodynamic efficiency of ground effect aircraft is nearly twice that of conventional aircraft when flying at approximately 20 feet above the water. The efficiency reduces the amount of fuel burned and provides complementary increases in payload capacity. Every foot of altitude is important for the ground effect aircraft's efficiency and operating costs.
Accordingly there is a need in the art for an altitude-measuring system that permits determination of a minimum flight altitude so that altitude may be measured with an error of only inches or a few feet. Furthermore, the altitude-measuring system should take into account variations in ocean waves and other ocean surface or terrain properties to permit accurate calculations of minimum safe flying altitudes. As a result, minimum safe altitudes can be applied to improve fuel efficiency and operating costs for large transport ground effect aircraft.